This experiment is one of the classics that is still a lot of fun to play with. It has to do with density. Density can be a difficult concept to grasp, but it is much easier once you have played with a density column.
Imagine that I have three cups that are the same size. If I fill one cup with water, one with sand, and one with lead. Would they all weigh the same? No, of course not. A cup of sand weighs more than a cup of water. Sand is denser than water. Lead is denser than sand, so a cup of lead weighs more than a cup of sand.
Density also explains why things float. A cubic inch of wood weighs less than a cubic inch of water, so the wood will float. The wood is less dense than the water. A cubic inch of lead weighs more than a cubic inch of water, so the lead will sink. Lead is more dense than water.
Now, what if you tried that with different materials, and different liquids?